Chlamydia trachomatis, Ureaplasma urealyticum and Mycoplasma hominis are frequently involved in gonococcal urethritis. We investigated 143 male White and Black patients with gonococcal urethritis (average age 22,5 years). Of these 29% had coexisting Chlam. trachomatis infection, 34% U. urealyticum infection and 13% Mycoplasma hominis infection. Conventional penicillin therapy did not affect Chlam. trachomatis, U. urealyticum or Mycoplasma hominis, which persisted in the lower urogenital tract, causing a so-called 'post-gonococcal urethritis.' Additional therapy with tetracycline or erythromycin was successful in most cases.